1. Field of the Invention
The present invention relates generally to satellite communications, and more particularly to a system and method to improve the operation of Global Positioning System (GPS) receivers in weak signal conditions.
2. Description of Related Art
During recent years location technologies have emerged as a promising research area with many high-impact applications in daily life. In particular, wireless operators around the world have identified Location Based Services (LBS) as an excellent opportunity for growth. Many of these applications are using Global, Positioning System (GPS) receivers for user position estimation. GPS receivers synchronize to satellite signals to estimate user-to-satellite ranges and apply trilateration methods for position computation. While GPS is designed for outdoor operation, recently many GPS receivers have been developed for indoor environments, where the satellite signals are significantly attenuated. Still these environments are very challenging for the receivers and their operation is not robust because of cross-correlation interference, multi-path presence, colored noise, etc.
The GPS receiver typically synchronizes to satellites in two steps: acquisition and tracking. Once the signal has been acquired, the receiver switches to tracking mode. Embodiments of this invention address the receiver operation during the tracking mode. In weak signal conditions, the receiver often loses the signal and the acquisition process has to be repeated. The loss of signal is very inconvenient as the acquisition process is time and resource consuming. In indoor and urban environments, the tracking process often fails due to the presence of strong multiple access interference (MAI) when strong signals jam weaker signals from the satellites. In such environments, the received signal powers may differ by more than 35 dB because of diverse propagation channels. GPS signals were initially designed for open-sky operation with cross-correlation interference immunity of about 20 dB, which is apparently not sufficient for many weak signal environments.
Conventional approaches use the same codes in the receivers and transmitters and they do not provide sufficient channel immunity in the presence of interfering strong signals. Advanced solutions modify despreading codes in the receivers in order to reduce the effects of MAI. For despreading code optimizations, one can use linear or non-linear approaches. The traditional non-linear interference cancellation techniques may not be applied to GPS receivers because of lack of training sequences, high computational loads and time delays. However, the linear receiver techniques may be feasible solutions. The decorrelator and minimum mean square error MMSE detectors have proven to be efficient techniques in decreasing the effects of the MAI in code division multiple access (CDMA) systems. The MMSE approach uses autocorrelation matrix inversion that is time and resource consuming. To simplify the processing, some methods estimate the cross-correlation signal and subtract it from the weak signal channel. This solution may not be optimal and computation overhead is still high especially in the presence of strong satellite signals. An ad-hoc solution has been suggested using an additional orthogonalization process for better separation of weak and strong channels. It has been observed that the performance of their method deteriorates with a few stronger signals and the method fails when there are more than four strong signals.